The invention relates to a plug for sealing a filler neck of a fuel tank with a cap which is dismountably attached to the end of the filler neck using a thread or bayonet, and with a sealing ring whose sealing surface faces a counter-sealing surface of the filler neck. A ring-shaped molding is provided between the sealing surface and the counter-sealing surface.
Naturally the cap of a fuel tank should be water-tight, especially that of an automobile. In addition, various forms of moldings are known to be found between a filler neck and plug or seal. By turning the cap, these are pressed together through the bayonet or thread and thus seal against leakage of liquid fuel. However, even when both are positioned correctly against the filler neck, there are still fuel vapors escaping. The vapors stream outwards when there is sufficient pressure gradient at the moldings of the sealing and are also diffuse through the sealing.
This diffusion through the sealing is also called permeation. It consists of fuel moleculesxe2x80x94either in liquid or gas statexe2x80x94penetrating the sealing at its surface, passing through the sealing and exiting at the surface to the outside world. In liquid a state, the molecular density is higher and the number of diffusing molecules larger (at the same temperate).
The general developing trend in the automotive industry tends towards decreasing emissions. For this reason, emissions by permeation are also being increasingly limited by law. Recently, the permeation rate is supposed to be limited to 3 milligrams of hydrocarbon per every 24 hours. This cannot be achieved with any molding, no matter how thoroughly constructed.
Thus, the invention is concerned with the task of creating a tank plug in such a way as to be able to adhere to this limitation. The invention achieves this by providing a sealing ring outside of the molding (as viewed from inside of the filler neck), the sealing ring being made of an elastomer with high resistance to permeation. The sealing surface and counter-sealing surfaces between the filler neck and plug create a narrow gap on the sealing ring side facing the molding, this gap extending to the sealing ring.
The addition of a second sealing ring is not merely a simple doubling. The molding ring protects the additional sealing ring from coming into contact with liquid fuel and consequently can be used as a vapor seal. For this, a material of high resistance to permeation can be chosen which is possibly less suitable for water sealants. At the same time, the gap reaching up to the sealing ring has yet another effect. It reduces the contact surface between the sealing ring and the fuel vapor to a minimum. Since the amount of diffusion is proportional to the contact surface, permeation is thus further reduced to a fraction of the amount.
Continuing along further with the idea of the invention, the sealing surface and the counter-sealing surface create an enlarged vapor chamber between the gap and the molding. This vapor chamber serves as a stabilizer chamber in which a stoppage of the fuel vapor flowing by the molding occurs. Thus, in the narrow gap directly before the sealing ring, a predominantly stream-free dead area is created in which the vapor concentration close to the sealing ring decreases so that even less vapor can diffuse into it.
An especially advantageous and easy to install arrangement of the sealing rings is achieved by having the sealing surface and counter-sealing surface created (at least partially) conically inwards. Also, this improves centering and preserves the molding, thus making the latter water-tight for a longer time.
In a preferred construction of the invention, the seal can be turned together with the cap yet is connected with it in such a way as to prevent them from falling apart. This prevents the seals from rubbing against the counter surface under pressure, when the seal is turned shut, which would cause wear to the seals.
In order to preserve and maintain the functionality of the sealing rings, it is especially advantageous if the upper end of the cap has a handle which is connected to the cap via a torque-limiting ratchet mechanism.
A further contribution to the sealing""s preservation consists of providing a ring-shaped lip seal at the bottom end of the cap which, together with a neck at the circumference of the filler neck, give a sealing effect. Thus, outside caked dirt is kept away from the sealing ring and molding. Finally, the sealing ring is preferably made of a fluorelastomer.